This invention relates to a method and an apparatus for operating a steering system for a motor vehicle.
Such a prior art steering system is disclosed in DE-OS 40 31 316 (corresponding to U.S. Pat. No. 5,205,371) and, to the extent that it is relevant to this invention, will be described with reference to FIGS. 1 and 2. In such a steering system, the motions of the actuator 13 or 23, the motor angle .delta..sub.M, are superimposed in the auxiliary drive 12 or 22 on the steering motions imparted by the driver through the steering wheel 11 or 21, the steering wheel angle .delta..sub.L detected by the sensor 28. The superimposed motion .delta..sub.L ' thus obtained is transmitted through the steering mechanism 14 and the steering linkage 16 to the steerable wheels 15a and 15b to set the steering angle .delta..sub.V. The actuator 13 or 23 can be an electric motor. The operating principle of such a servo steering system consists of providing that the steering can be accomplished very indirectly through the transmission i.sub.a of the auxiliary drive 12 or 22, and thus low steering wheel moments M.sub.L can be achieved. Because of this, very large steering wheel angles .delta..sub.L are avoided, inasmuch as suitable motor angles .delta..sub.M are superimposed, so that required output angles .delta..sub.L ' can be set with steering wheel angles of ordinary size according to the equation EQU .delta..sub.L '=.delta..sub.L /i.sub.a +.delta..sub.M.
The required motor angle .delta..sub.M required to assist steering, or its desired value, is determined from the steering wheel angle .delta..sub.L. In addition, the motor angle .delta..sub.M can also depend on signals Sm, with these signals Sm representing the vehicle motions detected by the sensors 26.
In a steering system there are usually one or two universal joints to connect the shafts of the steering mechanism and of the steering wheel with angular and longitudinal displacement. If there is only one universal joint (FIG. 3), it is well known that with a constant input speed .omega..sub.1 a pulsating output speed .omega..sub.2 is obtained according to the following equation: ##EQU1##
In this equation, .beta. is the angular offset of the joint and .alpha. is the angle of rotation of the input shaft.
It is likewise known that with two universal joints, the nonuniformity of rotation .omega..sub.2 /.omega..sub.1 is smoothed out by a known W- or Z-shaped arrangement, if the two angular offsets .beta..sub.1 and .beta..sub.2 are of equal magnitude. Other arrangements (FIG. 4) lead to nonuniform transmission of rotation, in the same way as a single universal joint.
However, transmission of the rotational steering motion has to be as uniform as possible for good steering precision; this requirement therefore limits the structural possibilities. Because of space limitations and other requirements for the arrangement of units in the engine compartment, under some circumstances no steering shaft design is possible that permits uniform transmission of motion.
The purpose of this invention is to compensate for structurally unavoidable nonuniformity of rotational steering motion.